JPH0586455A - Manufacture of high salt resistant steel for reinforcing concrete whose surface is coated with al alloy layer - Google Patents

Abstract

PURPOSE:To provide the method for manufacturing a high salt resistant steel for reinforcing concrete whose surface layer is coated with an Al allay layer. CONSTITUTION:A carbon steel is buried in mixed powder of, by weight, 1 to 42% Mg powder and the balance aluminum powder with inevitable impurities, which is heated at 800 to 1200 deg.C for 0.5 to 90min to enter Al of 5 to 20% weight average into solid soln. in the surface layer part to 10mum depth. In this way, the surface layer is coated with the Al solid soln. layer, by which the steel for reinforcing concrete excellent in salt resistance and cold workability can industrially be manufactured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a reinforcing bar or a PC steel material which is a concrete reinforcing material.

Since carbon is a strong alkaline environment in which the pH is usually higher than 10, carbon steel which corrodes in the atmosphere does not corrode either. However, in a concrete structure constructed in a beach area or a concrete structure in which unwashed sea sand is used for kneading sand or seawater is used for kneading water, a strong alkali having a pH of more than 10 is used. Also, due to the presence of Cl ⁻ ions, the steel material is corroded. As a result, there is a risk that the steel material has a reduced cross-sectional area and strength, and hydrogen generated by corrosion enters the steel and becomes brittle. Also,
Since the volume of corrosion products is larger than that of steel, cracking occurs in concrete, which promotes corrosion more and more and significantly shortens the life of concrete structures.

The present invention provides a method for producing a steel material having excellent so-called salt resistance, which is used as a reinforcing material for a concrete structure containing such Cl ⁻ ions.

[0004]

2. Description of the Related Art Conventionally, it has been customary to use river sand, which can be easily collected, for concrete and knead it with river water, industrial water or tap water. However, as a result of the dramatic increase in concrete structures, it is not easy to collect river sand, and the use of sea sand has been forced. When sea sand is used, it is necessary to perform sufficient washing with water to remove salt, which causes a drawback that the working process becomes complicated and the construction period becomes long. Further, in the case of an offshore structure, the invasion of Cl ⁻ ions into the concrete cannot be prevented, so that corrosion of the reinforcing steel material and consequent shortening of the concrete life cannot be avoided.

All of these drawbacks are caused by corrosion of the reinforcing steel material. Therefore, a reinforcing steel material having excellent corrosion resistance, that is, salt resistance in a strong alkaline environment containing Cl ⁻ ions can be solved.

Conventionally, as a reinforcing material for a concrete structure which requires salt resistance, a coated reinforcing bar coated with epoxy or Zn
Plated rebar and stainless rebar have been used. However, not only the coated rebar cannot be welded as it is, but also the coating film in the bent portion is easily peeled off and corrosion occurs, so that the expected effect is not always obtained. Zn-plated rebars and stainless steel rebars have correspondingly salt resistance and can be expected to be effective, but they have drawbacks that they are difficult to weld and expensive.

On the other hand, the Applicant has found that the steel to which a large amount of Al is added has very good salt resistance, and
Invented a steel material containing 7 to 20% by weight (Japanese Patent Laid-Open No. 64-7).
9346). This invention steel does not corrode even in a Ca (0H) ₂ suspension aqueous solution containing about 2% of Cl ⁻ ions and having a pH of 12 so that it is possible to manufacture a concrete structure using sea sand. However, since this steel is an alloy premised on containing a large amount of Al, the inevitable manufacturing defects could not be solved. That is, smelting in a converter, which requires decarburization by blowing oxygen, has a very poor yield and is difficult to mass produce. Further, since the hot ductility was extremely poor, huge cracks frequently occurred during casting and rolling, and it was impossible to process the reinforcing bars and PC steel materials. In other words, it could not be manufactured by the usual steel manufacturing process (melt refining-casting-rolling).

On the other hand, the present inventors have noticed that only the surface layer portion is required to have salt resistance, and a steel material having a surface layer portion coated with an Al solid solution layer of 5 to 15% by weight is used. Invented (Japanese Patent Application No. 2-138400). It was confirmed that this steel material has excellent ductility and toughness, and of course, has excellent salt resistance in a high pH environment. Industrial mass production of this steel material
A solid phase diffusion method that combines plating spraying and the like, and a cast clad in the casting stage are conceivable and can be implemented. But,
These methods are not always advantageous from the viewpoint of constructing an inexpensive manufacturing process, because they include special equipment and steps that require significant changes to conventional manufacturing steps.

The present inventors considered using a chemical reaction in combination in order to increase the efficiency of the method of forming the Fe-Al alloy on the surface layer portion.

A so-called calorizing method is known as a method for coating the surface of a steel material with an Al alloy layer by utilizing a chemical reaction. For example, pages 1163 to 1167 of the Metal Surface Technology Handbook (Metal Surface Technology Association) published in 1976 include:
A method of embedding a steel material in a mixed powder of an Al powder, an Al ₂ O ₃ powder as a buffer material, and a halide such as NH ₄ Cl as an activator to generate a Fe—Al intermetallic compound layer having a high Al concentration on the surface of the steel material is known. It is shown. However, since the calorizing method aims to improve the high temperature oxidation resistance, it is a technique in which the effect is not obtained unless the Al concentration of the coating alloy layer is usually 60% or more.

The inventors of the present invention tried to change the treatment conditions of the calorizing method so as to slow down the generation and diffusion of Al, and control the Al concentration to an appropriate level from the viewpoint of the decrease of Al concentration, that is, salt resistance. First, the treatment was terminated in a short time to reduce the Al concentration, but a target concentration portion was locally recognized, but the high concentration portion and the uncoated portion coexisted, and the Al in the planar direction coexisted. Uneven density was inevitable. Next, an attempt was made to lower the treatment temperature, but not only the required treatment time became longer, but also the unevenness of the Al concentration as in the case of the treatment for a short time was unavoidable. That is, in view of salt resistance, an appropriate Al concentration level, that is, 5%
It was impossible to control the Al concentration to 20% or less by the calorizing method.

[0012]

DISCLOSURE OF THE INVENTION The present invention is intended to form a surface alloy layer having an appropriate Al concentration uniformly and in a short time from the viewpoint of salt resistance, and is different from the conventional calorizing method. A steel material for reinforcing a concrete structure having excellent salt resistance having 5 to 20% Al alloy phase on the surface by clarifying chemicals and treatment conditions capable of creating different Al generation methods and appropriately controlling it. Is disclosed.

[0013]

The present inventors have found that Al powder,
A conventional diffusion coating method using Al ₂ O ₃ powder and a halide such as NH ₄ Cl was examined for a method of reducing the Al solid solution concentration with respect to Fe. In the calorizing method, a chloride such as NH ₄ Cl reacts with Al to generate an Al chloride, which decomposes on the surface of the steel material to generate Al. The reaction between chloride and Al and the decomposition of Al chloride on the surface of steel, that is, the rate of formation of Al on the surface of steel, is
It is larger than the diffusion speed of. As a result, the Al concentration on the surface of the steel material rapidly increases, and an intermetallic compound layer is formed. If the processing is terminated in a short time or the temperature is lowered,
Since the Al concentration determined by Al generation on the surface and diffusion into the steel is not in a substantially constant steady state, it is considered that the variation of the concentration in the plane direction was large as described above.

Therefore, an attempt was made to raise the treatment temperature in order to increase the diffusion rate of Al, but it is estimated that the generation rate of Al was also higher than the increase of the diffusion rate, and as a result, the solid solution of Al was formed. The concentration could not be lowered. Not only that, but a thick intermetallic compound layer, which is easily peeled off during bending, was generated, which was off the plan.

Next, in order to delay the formation of Al chloride,
Diffusion treatment was performed by reducing the amount of NH ₄ Cl mixed. However, when NH ₄ Cl was reduced, a target concentration portion was locally recognized, but unevenness of the Al concentration in the plane direction was unavoidable due to the presence of an uncoated portion. Further, diffusion treatment was performed only with Al powder and Al ₂ O ₃ powder without mixing NH ₄ Cl. In this case, when the treatment temperature is lower than the melting point of Al, it is practically impossible to form an Al solid solution layer.
The target Al concentration portion of 20% could not be formed.

The present inventors abandoned the diffusion treatment of Al through Al chloride used in the calorizing method, and made the reaction of relatively slow Al formation slow in relation to the diffusion rate of Al in steel. I considered using it. As a result, Al ₂ O ₃
It was found that the method of producing Al by the reduction reaction of Mg with Mg is appropriate. According to this method, since there is no rapid increase in Al in the surface layer portion, an intermetallic compound was not generated even when high temperature treatment was performed, and an alloy layer having a target Al degree of 5 to 20% could be formed in a relatively short time.

The present invention has been completed based on the above findings. That is, carbon steel was embedded in a mixed powder of Mg powder 1 wt% or more and 42 wt% or less, the balance unavoidable impurities and Al ₂ O ₃ powder, and 800 ° C. or more and 1200
It is heated at a temperature of ℃ or less for 0.5 min or more and 90 min or less, and the surface layer up to a depth of 10 µm has a weight average of 5% or more
% Or less of Al dissolved in the surface layer
Disclosed is a method for producing a high salt-resistant concrete reinforcing steel material coated with an alloy layer.

Next, the reasons for limitation of the present invention will be described. Since Al produced on the surface of the steel material is produced by the reduction of Al ₂ O ₃ with Mg, the Al concentration of the Al alloy layer produced on the surface layer of the steel material varies depending on the amount of Mg as a reducing agent. When the mixing amount of Mg is less than 1% by weight, the Al concentration of the Al alloy layer is less than 5% and salt resistance cannot be obtained. Therefore, the lower limit of the mixing amount of Mg is set to 1% by weight. On the other hand, over 42% M
Even if g was mixed, there was no increase in the generation rate of the Al alloy layer, and the advantage of increasing the concentration was not recognized, so 42% was made the upper limit.

The heat treatment temperature directly affects the diffusion treatment time required to obtain an alloy layer having a predetermined thickness. That is, a high temperature requires a short time and a low temperature requires a long time. 800 ° C
Is less than 5% to 20% of the Al alloy layer from the outermost surface to a depth 1
Since a diffusion treatment time of more than 1 hour is required to form Oμ, 800 ° C. was set as the lower limit temperature. On the other hand, 12
If the temperature exceeds 00 ° C, the crystal grains of the base material become coarse and the toughness deteriorates, and the quality as a reinforcing bar or PC steel cannot be maintained, so 1200 ° C was set as the upper limit.

The heat treatment time is limited by the desired Al concentration, depth and heat treatment temperature. However, if it is set to less than 0.5 min for a short time treatment at a high temperature, the Al concentration and the thickness of the solid solution layer become uneven, so the lower limit is set. On the other hand, 90min
If it exceeds, the crystal grain of the base material becomes coarse and the toughness deteriorates even if the treatment is performed on the low temperature side, and the quality as a reinforcing bar or a PC steel material cannot be maintained. Therefore, the upper limit was set to 90 min.

In the present invention, it is possible to stir the diffusion treatment agent with an inert gas or nitrogen so that the mixed powder does not stay on the surface of the steel material. By this stirring treatment, there is no local unevenness in the mixing ratio of Mg and the temperature is made uniform, so that the Al concentration and thickness of the alloy layer formed in the steel material are made uniform.

[0022]

According to the method of the present invention, the surface of a steel material is coated with Al produced by reducing Al ₂ O ₃ with Mg, and the rate of Al production and the diffusion rate of Al in Fe are controlled over a wide temperature range. From the balance, the steel surface is 5 to 2
It is possible to stably generate a 0% Al solid solution layer. Furthermore, the method of the present invention can produce a low-concentration Al alloy layer in a short time without generating an intermetallic compound even when the diffusion coating treatment is performed at a higher temperature than the conventional method. By the method of the present invention, a steel material coated with an Al solid solution layer having an Al solid solution amount of 5 to 20% can be easily manufactured. The reinforcing bar and PC steel material according to the present invention have excellent salt resistance in an alkaline environment and have strength, ductility and toughness as a reinforcing material for a structure for reinforced concrete.

[0023]

[Example] Diameter 25 mm specified in JIS G3112
Steel bar SD30B steel for reinforced concrete of No. 1 was hot-rolled and then pickled to remove scales, and then Al in various diffusion treatment agents.
Diffusion penetration treatment was performed. Table 1 shows the composition of the main component of the diffusion treating agent and the production conditions such as the temperature and time of the diffusion permeation treatment. The diffusion treatment agent was stirred with an inert gas or nitrogen so that the diffusion treatment agent did not stay in the surface layer of the steel material.

Table 1 shows the thickness of the Al solid solution layer produced under these conditions, the surface solid solution concentration of Al, the occurrence of cracks due to 2D-180 ° bending, and the result of the salt resistance test. For salt resistance, reinforced concrete made from the test material prepared by kneading with salt water containing 0.8% NaCl was used.
A reinforcing bar corrosion test in concrete was performed using an autoclave apparatus specified in SA6205, and the rust generation area ratio at that time was measured and evaluated.

The present inventions 1 to 6 include 1% by weight of Mg powder.
Since the average Al concentration of the Al solid solution layer was set to 5% or more and 15% or less by using the diffusion treating agent containing 42% or less, excellent salt resistance was exhibited in an alkaline environment and the peel strength of the coating was good. On the other hand, Comparative Example 7 uses Al powder as the diffusion treatment agent,
Since Al ₂ O ₃ and NH ₄ Cl were used, the solid solution concentration of Al increased, and not only the salt resistance in an alkaline environment was poor, but also the coating was damaged by bending.

[0026]

[Table 1]

[0027]

As described above, according to the present invention, it becomes possible to coat an Al solid solution layer having excellent salt resistance and a small Al solid solution concentration on a steel material with good adhesion in a short time. As a result, it has become possible to manufacture a steel material having excellent salt resistance in an alkaline environment at a relatively low cost.

The steel material of the present invention can be used not only as a reinforcing bar for a concrete structure which is affected by salt content, but also in a place where salt resistance is required in an alkaline environment.

Claims (1)

[Claims] 1. Carbon steel is embedded in a mixed powder of 1 wt% or more and 42 wt% or less of Mg powder, the balance unavoidable impurities and Al ₂ O ₃ powder, and 0.5 min or more at a temperature of 800 ° C. or more and 1200 ° C. or less. Manufacture of a high salt-resistant concrete reinforcing steel material having an Al alloy layer coated on the surface layer, characterized in that 5% or more and 20% or less by weight average of Al is solid-dissolved in the surface layer portion up to a depth of 10 μm by heating for 90 min or less. Method.